i am almost done with the 3rd movie\year and have unlocked over 20 characters but still somehow dont have 1 single Slytherin person. WTF? are they just hard to unlock or do you have to beat the game before you can get them? it seems stupid that you can get someone from all the other houses really easily.

Yes. As soon as I got Reducto in 4th year I went back to the spider level for Lucious Malfoy so that I had a dark magic character. Several slytherin can be found after that as well. Look for the character guide posted here to search for specific people if needed.